1. Technical Field
The disclosed embodiments are directed generally to fluid power systems employing over-center pump/motors, and, in particular, to hydraulic hybrid vehicles.
2. Description of the Related Art
In configuring a series hydraulic hybrid vehicle, typically there will be at least one pump/motor acting primarily as a pump to convert mechanical power to hydraulic power, and at least one other pump/motor acting primarily as a motor to propel the vehicle. There arises the problem of packaging these units in a way that is economical in terms of cost, weight, and volume. In particular, each pump/motor must be provided with a high-pressure connection and a low-pressure connection. These are usually provided by heavy-duty hydraulic hoses that are bulky, stiff, and difficult to route past other components on the vehicle.
Further, in the development of any motor vehicle, economy of scale is always sought in order to minimize cost of production. No matter what components are employed, the market size for a given type of vehicle determines the degree to which production quantities can result in sufficient economy of scale to make the vehicle marketable to its intended users.
Hydraulic hybridization is particularly well suited to heavy-duty vehicles, due to the high power capability afforded by hydraulic drive components and the fact that such vehicles consume a lot of fuel. However, in contrast to light-duty passenger cars, which are sold in large quantities to the general public, the market for heavy-duty vehicles such as trucks, buses, and industrial vehicles is fragmented into relatively small segments of relatively low production volume. Each segment may in turn include many distinct specialty products produced in small quantities. Often, products such as buses and delivery vehicles are produced as generic chassis packages by one manufacturer and are then sold to a variety of aftermarket vendors to be fitted with specialty coachwork suited to a specific niche market or application.
This market fragmentation puts heavy-duty vehicles at an economic disadvantage for hybridization. It makes it unlikely that any single model of heavy-duty vehicle can achieve sufficient production quantities to allow large economies of scale to develop for the specialty components needed for its hybridization.
This problem could be alleviated by providing a modular hydraulic hybrid drivetrain that can be installed in a diverse variety of individual heavy duty vehicle models. Production volumes for the components in the modular drivetrain could then be expanded across multiple vehicle product lines, allowing economies of scale to develop more effectively, at least for the modular drivetrain.
Another goal to reduce hydraulic hybridization costs would be the elimination of many of the hoses and external fluid connections presently necessary for fluid routing. By bringing most fluid routing into the interior of the device, parts count and assembly labor would be reduced, while safety, simplicity, and ease and cost of installation would be improved.
The goal of a practical and affordable modular hydraulic drivetrain presents many difficult technical hurdles. Aside from the task of effectively integrating the necessary components, it would be necessary to maintain ease of installation by a variety of line personnel across a variety of chassis configurations. A modular hydraulic drivetrain, with its accumulators and auxiliary connections, would have to be easily transportable as a unit to any assembly factory in any part of the world, and easily transported within the factory. It would also be necessary to minimize installation time to be compatible with the movement of an assembly line. Because fluid handling capability is often not present on a vehicle production line that does not normally install hydraulic components, it would be preferable to provide a pre-plumbed, fully primed unit. To be applicable to the broadest possible number of host chassis, the structural and volumetric requirements for the host chassis and the degree of custom adaptation necessary should be minimized.
In U.S. Patent Application 2009/0172942 (Hindle), “Integrated Hydraulic Hybrid Drive Module and Method of Installing Same”, there is disclosed a method of providing a modular hydraulic hybrid drivetrain by packaging the necessary drivetrain components on a modular platform that can be installed permanently on a vehicle chassis. A product made after this description has been offered commercially by Parker Hannifin Corporation. In contrast with the present invention, Hindle does not place the engine pump and drive motor(s) in a common case. In addition, the Hindle application is oriented to a permanent support structure that becomes part of the vehicle, and is mounted primarily to one side of a vehicle frame. The preferred embodiment of the present invention is instead oriented toward a removable support structure that does not become part of the vehicle, and carrying a modular series powertrain that mounts near the center of the vehicle in a position similar to a normal vehicle driveline.